Debasis Sen

Evaporation Driven Self-Assembly of a Colloidal Dispersion during Spray Drying: Volume Fraction Dependent Morphological Transition

Morphological transition of droplets during evaporation driven self-assembly of colloidal dispersion of alumina particles has been investigated. It was found that a sphere to doughnut-like transition of the droplet morphology takes place even when the rate of drying remains moderate and is not extremely fast. Further, it has been seen that such transition is strongly dependent on the volume fraction of the colloids in the droplets. The transition proceeds via buckling of the initial spherical droplets, which occurs when the capillary forces driving the deformation overcomes the interparticle electrostatic forces. However, the transition is hindered and the buckling probability is reduced due to the inherent spatial constraint when the colloid volume fraction is increased. Mesoscopic structures of the assembled grains have been investigated by scanning electron microscopy, small-angle neutron scattering, and dynamic light scattering techniques. Interestingly, it has been observed that the functionality of photoluminescence spectrum of the dried nanoporous grains depends somewhat on the grain morphology.

Formation and characterization of highly crosslinked anion-exchange membranes

Highly crosslinked/hyperbranched anion-exchange membranes have been prepared by anchoring poly(vinylbenzyl chloride) (PVBCl) within the pores of poly(propylene) microporous base membranes by in situ crosslinking of PVBCl with a diamine 1,4-diazabicyclo[2.2.2]octane (DABCO). The resulting PVBCl-filled precursor membranes were converted to anion-exchange membranes by reacting these with (i) excess of DABCO followed by alkylation with α,α′-dibromo-p-xylene (DBX) (membrane A), and (ii) with excess of tetraethylenepentamine (TEPA) (membrane B). A third membrane C was synthesized by alkylating membrane B with DBX. The chemical analyses indicated that these anion-exchange membranes consist of highly crosslinked/hyperbranched anionic gels within the pores of host microporous membranes. These anion-exchange membranes were characterized in terms of water-uptake capacities, ion-exchange capacities and thermal stability. The physical structures of the membranes were examined by small angle X-rays scattering (SAXS) analysis. The study of SAXS profiles of the dry and water equilibrated membrane A samples indicated that microstructure of anionic gel within the pores of membrane was changed significantly on water equilibration. However, no significant change in the SAXS profile was observed in wet samples of membranes B and C with respect to their dry samples. Thus, the crosslinking generated in membrane A was flexible and very rigid in membranes B and C. The self-diffusion coefficient of I− ions and transport numbers of Cl− ions were measured to examine the effects of crosslinking on transport properties of the membranes.

Origin of Buckling Phenomenon during Drying of Micrometer-Sized Colloidal Droplets

The origin of the buckling of micrometer-sized colloidal droplets during evaporation-induced self-assembly (EISA) has been elucidated using electron microscopy and small-angle neutron scattering. Doughnut-like assembled grains with varying aspect ratios are formed during EISA at different physicochemical conditions. It has been revealed that this phenomenon is better explained by an existing hypothesis based on the formation of a viscoelastic shell of nanoparticles during drying than by other existing hypotheses based on the inertial instability of the initial droplets and hydrodynamic instability due to thermocapillary forces. This conclusion was further supported by the arrest of buckling through modification of the colloidal interaction in the initial dispersion.

Barium, calcium and magnesium doped mesoporous ceria supported gold nanoparticle for benzyl alcohol oxidation using molecular O2

In the era of sustainable energy, catalysis using gold nanoparticles has drawn considerable attention from world researchers. Oxidation of benzyl alcohol by molecular O2 is an atom efficient path to synthesize benzaldehyde. Nanocrystalline ceria has been proven as a useful support to disperse gold nanoparticles since last few years, however there are a few reports on mesoporous ceria supported gold nanoparticles. In this work a systematic investigation was carried out to improve the activity of Au/CeO2 catalyst by incorporating Ba2+, Ca2+ and Mg2+ cations into the ceria lattice through a sol–gel procedure. Both the doped ceria and ceria supported gold nanoparticles are characterized by BET S.A, XRD, TEM, SAXS, XPS, TPR, CO2-TPD techniques. BET S.A measurements show the mesoporous oxides where H3 hysteresis loops are found. The decrease in the crystallite size of ceria after doping by metal cations is observed in the XRD measurement. The TEM and HRTEM characterization shows the nanocrystalline particle size around 30–50 nm and gold nanoparticles around 10–15 nm in size. Distribution in the particle size for doped ceria have been obtained using SAXS measurements where narrow distributions of ceria particles are found in the 10–20 nm range. The existence of oxide vacancies and the mixture of Ce3+/Ce4+ oxidation states are observed for doped ceria materials in the XPS investigation. The strong gold-support interaction was also evidenced by XPS characterization where oxidic gold was found on the doped ceria surface. Lowering of the reduction peak in ceria after gold nanoparticle deposition was observed from TPR investigation whereas the change in basic site distribution is observed from CO2 TPD experiment, instigating new insights into the surface properties of the catalysts. The catalytic activities of the catalysts were determined for benzyl alcohol oxidation reactions using molecular O2. The catalytic activity was in the order of Au/Ba–CeO2 > Au/Ca–CeO2 > Au/Mg–CeO2 > Au/CeO2. The synergistic effect of gold nanoparticles and dopant cations to the ceria was explained in this work.

Evidence of clustering in an aqueous electrolyte solution: a small-angle X-ray scattering study

The work reported here was motivated by a desire to verify the existence of structure – specifically MP-rich clusters – induced by sodium bromide (NaBr) in the ternary liquid mixture 3-methylpyridine (MP) + water (W) + NaBr. We present small-angle X-ray scattering (SAXS) measurements in this mixture. These measurements were obtained at room temperature (∼298 K) in the one-phase region (below the relevant lower consolute points, TLs) at different values of X (i.e., X=0.02−0.17), where X is the weight fraction of NaBr in the mixture. Cluster-size distribution, estimated on the assumption that the clusters are spherical, shows systematic behaviour in that the peak of the distribution shifts towards larger values of cluster radius as X increases. The largest spatial extent of the clusters (∼4.5 nm) is seen at X=0.17. Data analysis assuming arbitrary shapes and sizes of clusters gives a limiting value of cluster size (∼4.5 nm) that is not very sensitive to X. It is suggested that the cluster size determined may not be the same as the usual critical-point fluctuations far removed from the critical point (TL). The influence of the additional length scale due to clustering is discussed from the standpoint of crossover from Ising to mean-field critical behaviour, when moving away from the TL.

Pore morphology and pore surface roughening in rocks: a small-angle neutron scattering investigation

Pore morphology and pore-matrix interface roughening in some metamorphosed sedimentary rocks, sandstones and igneous rocks have been investigated using small-angle neutron scattering (SANS), in the length scales of ∼20–1000 nm., which reveal the fractal nature of the rock-pore interfaces. Surface fractal dimension of the metamorphosed rocks and the sandstones has been estimated to be ∼2.8 while, that for the igneous rocks has been found to be ∼2.3. An attempt has been made to explain the relatively high surface fractal nature of the former rocks with the help of a computer simulation model based on the formation mechanisms of these rocks. SANS data indicate some ideas about the upper cut-off of the fractal geometry for the igneous rocks as well as for the sandstone, but no unambiguous cut-off value has been obtained for the metamorphosed rocks in the accessible length scale. The multiple scattering effect in these rock specimens has also been looked into by performing the SANS experiments for the two thicknesses on each specimen.

Nanocomposite silica surfactant microcapsules by evaporation induced self assembly: tuning the morphological buckling by modifying viscosity and surface charge

Nanocomposite microcapsules of silica and surfactants have been synthesized using evaporation induced self-assembly through spray drying. It was established using electron microscopy and small-angle neutron/X-ray scattering experiments that the viscosity of the virgin dispersion and surface charge of colloidal components play a significant role in the buckling of spray droplets during drying. Hollow spherical grains are realized at relatively low viscosity and higher surface charge while mushroom like grains manifest at higher viscosity and lower surface charge. In the intermediate conditions, deformed doughnut shaped microcapsules are obtained. Scattering experiments establish the presence of the organization of micelle like aggregates of surfactants in the dried grains and also corroborate with the observations from electron microscopy. A plausible mechanism regarding the chronological pathways of morphological transformation is illustrated. Computer simulation, based on buckling of an elastic shell using a surface evolver, has been attempted in order to corroborate the experimental results.

Control of Buckling in Colloidal Droplets during Evaporation-Induced Assembly of Nanoparticles

Micrometric grains of anisotropic morphology have been achieved by evaporation-induced self-assembly of silica nanoparticles. The roles of polymer concentration and its molecular weight in controlling the buckling behavior of drying droplets during assembly have been investigated. Buckled doughnut grains have been observed in the case of only silica colloid. Such buckling of the drying droplet could be arrested by attaching poly(ethylene glycol) on the silica surface. The nature of buckling in the case of only silica as well as modified silica colloids has been explained in terms of theory of homogeneous elastic shell under capillary pressure. However, it has been observed that colloids, modified by polymer with relatively large molecular weight, gives rise to buckyball-type grains at higher concentration and could not be explained by the above theory. It has been demonstrated that the shell formed during drying of colloidal droplet in the presence of polymer becomes inhomogeneous due to the presence of soft polymer rich zones on the shell that act as buckling centers, resulting in buckyball-type grains.

One-step fabrication of thermally stable TiO2/SiO2 nanocomposite microspheres by evaporation-induced self-assembly.

The evaporation-induced self-assembly of mixed colloids has been employed to synthesize microspheres of TiO(2)/SiO(2) nanocomposites. Small-angle neutron/X-ray scattering and scanning electron microscopy experiments reveal the hierarchical morphology of the microspheres. Although the internal structure of the microspheres, consisting of solely silica nanoparticles, gets significantly modified with time because of the reduction in the high specific surface area by internal coalescence, the same for the composite microspheres remains stable over an aging time of 1 year. Such temporal stability of the composite microspheres is attributed to the inhibition of coalescence of the silica nanoparticles in the presence of titania nanoparticles. X-ray diffraction and thermogravimetric results show the improved thermal stability of the composite grains against the anatase-to-rutile phase transition. Such thermal stability is attributed to the suppression of the growth of titania nanoparticles in the presence of silica nanoparticles. The UV-vis results indicate the confinement effect of the TiO(2) nanoparticles in the silica matrix. A plausible mechanism has been elucidated for the formation of microspheres with different morphology during self-assembly.

Evaporation-induced self assembly of nanoparticles in non-buckling regime: volume fraction dependent packing.

Hierarchically structured micrometric spheres are synthesized by evaporation-induced self assembly of silica colloids using spray drying technique. Packing of nanoparticles during drying of droplets is an important issue. The motivation of the present work is to investigate the effects of concentration of initial colloidal dispersion on the packing of the nanoparticles in assembled grains in non-buckling regime of drying. It has been observed that the packing of nanoparticles inside the dried grains, even in the non-buckling regime, varies significantly with concentration. Although, the packing of nanoparticles remains uniform in an assembled grain at smaller concentration, the same becomes non-uniform at higher concentration. Further, the average packing fraction of the nanoparticles within the assembled grains, decreases with increasing colloidal concentration. These observations have been attributed to the modification in viscosity of the initial dispersion. Electron microscopy, light scattering measurements have been performed to probe overall morphology of the dried grains, while inter-particle correlation inside the grains has been investigated by small angle neutron scattering.